The invention relates to a contact element for a varistor.
Varistors are known from the prior art.
Varistors provide a voltage-independent resistance in electrical circuits. Varistors are therefore used in a wide range of applications, typically in order to discharge overvoltage above a certain threshold voltage, thus preventing the overloading or damaging of a subsequent device. One example of such overvoltage is voltage that can occur as a result of lightning.
The varistor generally contains a granular metal oxide, e.g., zinc oxide and/or bismuth oxide and/or manganese oxide and/or chromium oxide and/or silicon carbide, which is almost always inserted in the form of (sintered) ceramic between two planar electrodes as supply elements ZL1, ZL2. One exemplary varistor VAR is shown in FIG. 1. It has a first supply line ZL1 on one side and a second supply line ZL2 on an opposing side.
Typically, the individual grains possess varying conductivity. Boundary layers are formed at the respective grain boundaries, that is, at the contact points of the grains. It can be determined that, as the thickness increases, the number of grain boundaries increases, and hence the threshold voltage as well. If voltage is applied to the supply elements ZL1, ZL2, an electrical field is formed. Depending on the voltage, the boundary layers are broken down and the resistance decreases.
Due to the material characteristics of the varistor, neither the distribution of current nor the breakdown of the boundary layers is a uniform process; rather, localized current paths are formed, for example current paths S1, S2, that reach the conductive state at different speeds. For example, in FIG. 1, the current path S1 becomes conductive more quickly than the current path S2, since a lower voltage (200 V, for example) needs to be overcome on the current path S1 than on current path S2 (300 V, for example).
As a result of the material characteristics, and due to the use of the varistor, leakage currents occur. While these leakage currents are very usually small, they can lead in some circumstances to substantial heating of the component, thus posmg a fire hazard. To counteract this, a temperature sensor is typically used which actuates a switch TS when a certain temperature is exceeded. This is shown, for example, in FIG. 2. However, temperature sensors can only be used to detect slow events. Quick heating such as that which occurs when a high voltage is applied, for example, leads to a greatly delayed rise in temperature at the temperature sensor due to the necessary and known slow heat conductance, so that the varistor would generally already be destroyed. The selectivity is also generally limited here; that is, only small currents can be cut off.
Such an energy input can occur, for example, as a result of overvoltage occumng over an extended period, thus leading to an interconnection of the varistor VAR, upon which the short-circuit current of the network is discharged via the varistor. In this case, substantial heating of the varistor VAR occurs, and there is a fire hazard. Furthermore, the varistor VAR can be damaged in this way to the extent that the varistor is explosively shorted out.
Typically, varistors VAR are therefore provided with an upstream fuse F that is dimensioned such that the maximum impulse current load Im of the varistor VAR can still be discharged, but a cut-out is brought about upon exceeding of the maximum impulse current load Im. However, a high impulse current capacity of the fuse is always also associated with a high fuse rating. That is why an interruption of the (starting) short-circuit current only occurs comparatively late in the event of a fault.
Nonetheless, damage occurs in varistors VAR time and time again that cannot be detected by the abovementioned elements, that is, currents occur that can no longer be shunted off by the selectivity of the thermal cut-out TS but that are too small for an upstream fuse F.